During development, neurons find and synapse with one another in a remarkably precise way. How the nervous system becomes properly "wired" during development is one of the major unsolved problems in biology. The unfolding of neuronal specificity involves a series of highly specific recognition events between individual neurons. The long term goal of this research program is to understand the cellular and molecular basis of cell recognition during neuronal development. Extensive cellular studies in the grasshopper embryo have shown that individual growth cones express selective affinities for particular axonal surfaces, and that these specificities give rise to the stereotyped patterns of selective fasciculation. Experimental results have suggested that many different molecules are differentially expressed on the surfaces of embryonic growth cones and axon fascicles. Monoclonal antibodies (MAbs) have revealed cell surface antigens whose temporal and spatial expression in the embryo corelate with these predictions. Whereas the grasshopper embryo is an excellent model system for a cellular analysis of neuronal development, the Drosophila embryo has obvious attributes for a molecular genetic analysis. Recent studies have shown that the early Drosophila embryo is a minature replica of the grasshopper embryo in terms of its identified neurons, their growth cones, and their selective fasciculation choices, thus opening the way for a combined cellualr and molecular genetic analysis of cell recognition. This proposal is to study the molecular basis of cell recognition during neuronal development in the Drosophila embryo. The specific aims are (i) to isolate cDNA clones encoding molecules expressed on specific subsets of embryonic neurons early in development; (ii) to analyze the structure, distribution, and localization of the mRNAs and proteins they encode; and (iii) to understand the function of these surface molecules during neuronal development. The cDNA clones will be isolated by subtraction cDNA cloning, experssion cDNA cloning using specific MAbs, and/or synthetic oligonucleotide probes to amino acid sequences of purified antigens. The molecules will be characterized using recombinant DNA techniques, in situ hybridization, antibodies generated against their encoded proteins, primary cell cultures, and transfected cell lines. The function of these molecules will be studied using genetic analysis, gene fusions, and gene transformation techniques.